A sensor is a device that detects a specific physical quantity belonging to the sensor environment, through the means of one or more detection mechanisms which converts the physical quantity to a sensor specific output signal. Well known examples of often used sensors include thermometers, speedometers, voltmeters, radars and seismometers. The vast majority of modern mechanical sensors are made using microelectromechanical system (MEMS) technology, as MEMS allows for small scale sensors and inexpensive mass production. With the development of MEMS technology and the advent of micro optical sensing technologies the expanded term microopto-electromechanical systems (MOEMS) has become widely accepted. MOEMS allows for much more versatile sensor designs and detection of quantities in more difficult accessible environments than previous technologies.
All-optical sensors have a number of advantages that make them interesting for a broader range of applications, such as the low transmission loss in optical fibers which enables remote sensing. The sensing method of all-optical sensors can in general be divided into amplitude modulated (AM) and frequency modulated (FM) sensing. Frequency modulation refers to the fact that changes in wavelength of the reflected light are used for determining the physical quantity measured, rather than the amplitude of the reflected light as in amplitude modulation. While AM based sensors can achieve extremely high sensitivities due to the on/off characteristics of the signal, the signal is inherently vulnerable to transmission loss and noise and they are not easily integrated into larger sensor arrays since each sensor requires its own transmission line. On the other hand, FM based sensors do not have nearly the same sensitivity as AM based sensors, however, they are much more robust with respect to noise and are easily integrated into large distributed sensor systems as multiple sensors can share one transmission line. The dominating all-optical sensor today is the fiber Bragg grating (FBG) sensor based on frequency modulation and where the deformation of a Bragg grating is used for modulating the signal.
There are four major advantages of optical sensors: They exhibit immunity to electromagnetic interference, they are resistant to harsh environments, they simplify the process of measuring distributed physical quantities e.g. pressure, temperature and stress, and they are capable of multiplexing. However, compared to the vast number of electrical and opto-electrical sensors available today, all-optical sensors currently represent a small niche, since most all-optical sensors cannot in general compete with their electrical counterparts when it comes to sensitivity, dynamic range or price.